Temperature and thermoclines
0 of 4 hands-on
01 Introduction
A diver can go from comfortable to cold in a few metres. Temperature in the water is not even. It forms layers, and the boundary between them can be sudden and obvious. Understanding these layers helps you choose the right exposure suit, plan the dive and spot changes in visibility and marine life.
Objectives
- Describe the layers of temperature in the sea and in lakes
- Explain what a thermocline is and how a diver notices it
- Explain why water chills a diver faster than air and how to choose thermal protection
02 Watch
Layers of warmth
4 min · Video in production
- 1
A diver descending through clear warm water into a visible shimmering layer, with the thermometer on a dive computer falling.
- 2
A cross-section of a lake in summer, with a warm top layer, a thermocline and a cold bottom layer.
- 3
A comparison of heat loss in air and in water, shown as arrows leaving a body.
- 4
A diver choosing between a shorty, a 5 mm wetsuit and a drysuit.
03 Learn
Three layers
In many places, the sea in summer is made up of three layers. The sun warms the top layer, and wind and waves mix it, so it has a fairly even temperature. Beneath it is a zone where the temperature falls quickly with depth. This is the thermocline. Below that, the water is cold and changes little.
| Layer | Description |
|---|---|
| Mixed layer | Warm, well-mixed surface water, with a fairly even temperature |
| Thermocline | A zone in which the temperature falls rapidly with depth |
| Deep layer | Cold water, with little change in temperature |
The thermocline can be a few metres thick and a drop of several degrees. A diver sometimes sees it as a shimmering, blurred layer, a bit like heat haze, and feels it as a cold band. In lakes it forms in summer and is lost in autumn when the surface cools and the water mixes. In the tropics a thermocline can be permanent. In some places upwelling brings the thermocline close to the surface.
A layer of fresh water over salt water is called a halocline, and it can be seen in the same way. Both are layers where water of different density meets.
Predict first
Why does warm water float above cold water?
Why water makes you cold
Water conducts heat away from the body about 25 times faster than air at the same temperature. You can feel comfortable in air at 22 degrees and become chilled in water at the same temperature. Heat loss also gets worse with depth because a wetsuit compresses and insulates less, and because breathing dry, cold gas from the cylinder cools you from inside.
| Exposure protection | Typical use |
|---|---|
| Rash vest or lycra | Very warm water, sun and sting protection |
| Shorty or 3 mm wetsuit | Warm water, roughly above 25 degrees |
| 5 to 7 mm wetsuit | Cool to temperate water |
| Semi-dry suit | Cold water, with seals to limit flushing |
| Drysuit with undergarment | Cold water. It keeps you dry, and needs specific training |
Signs of getting too cold
- Shivering and goose bumps.
- Numb hands and feet, and clumsy fingers.
- Poor concentration and slow reactions.
- A blue tinge to lips and fingertips.
What thermoclines mean for the dive
- Visibility often changes at the thermocline, because the layers hold different amounts of plankton and sediment.
- The life you see may change from layer to layer.
- A drysuit or thick wetsuit may change your buoyancy as you pass through a thermocline, so check your weighting and trim.
- You can plan a dive to stay above a thermocline to keep warm, or go below it to find clearer water, if the plan permits.
Take a guess first
Water at 15 degrees takes heat from your body about 25 times faster than air at 15 degrees. If you lose heat from your body at 100 units per hour in air, about how many units per hour might you lose in water?
04 Explore
Try it yourself
Match each layer or term with its description.
Match each term with its description.
- Mixed layer
- Thermocline
- Halocline
- Drysuit
- Shivering
05 Apply
Scenario
A diver in a thin wetsuit descends through a thermocline at 15 m. The water drops several degrees, and after ten minutes the diver is shivering and fumbling with their gauge. The buddy has noticed.
What is the best action?
06 Check
07 Summary
Key points
- The sea is often layered: a warm mixed layer, a thermocline and cold deep water.
- Water takes heat from the body about 25 times faster than air.
- Choose suit thickness for the water, the dive and the individual. Add a hood and gloves in cold water.
- Shivering is a reason to end the dive.
Before you apply this in the water
- Have I been trained by a qualified instructor for this?
- Am I medically fit to dive?
- Do I have a buddy and a plan?
- Am I diving within my training and limits?
Answer every question in the Check section to mark this lesson complete.